Aims: To study the effect of a quaternary ammonium biocide, didecyldimethylammonium chloride ( DDAC), on the treatment efficiency of laboratory-scale rotating biological contactors (RBCs) as well as their component biofilms. Methods and Results: Biofilms were established on the RBCs and then exposed to 0-160 mg l(-1) (p.p.m.) DDAC at a flow rate of 2.5 l h(-1). The treatment efficiency of the RBC and the microbial activity of the biofilms were markedly decreased when 40 mg l(-1) DDAC or greater were applied to the units. However, DDAC had no effect on the number of viable bacteria in the biofilms when DDAC concentrations up to 80 mg l(-1) were applied to the RBCs. No viable bacteria could be detected in the biofilm when DDAC was applied at 160 mg l(-1). Extended observation over a further 40 d with 20 and 80 mg l(-1) DDAC showed similar results in terms of chemical oxygen demand removal, ATP content and viability of biofilms compared with those values over the first 12 d of exposure. Conclusions: There was at least a fourfold difference in the susceptibility of planktonic and sessile bacteria to DDAC. Cells acclimatized to DDAC did not increase their capability to degrade normal carbon sources or DDAC under the conditions used in this study. Significance and Impact of the Study: The results show that RBCs can be used to treat effluents containing DDAC at concentrations up to 20 mg l(-1) and that 160 mg l(-1) of DDAC was required to eliminate cells in established biofilms.